The present invention relates to a color image forming apparatus of an electrophotographic system having a drum-shaped photoreceptor provided therein with a plurality of imagewise exposure means, wherein a plurality of developing units are arranged around the circumferential surface of the photoreceptor so that toner images are formed and superposed while the photoreceptor makes one turn.
There have been known the following apparatuses as a method of forming a multi-color image:
an apparatus (A) in which photoreceptor drums, charging devices and developing devices each corresponding in number to colors necessary for forming the multi-color image are provided, and a mono-color toner image formed on each photoreceptor drum is superimposed one after another on an intermediate transfer member so as to form a color image, PA1 an apparatus (B) in which a single photoreceptor drum makes plural turns so that the charging, imagewise exposure and developing processes for each color are repeated so as to form a color image, and PA1 an apparatus (C) in which the charging, imagewise exposure and developing processes are conducted for each color sequentially while a single photoreceptor drum makes one turn so as to form a color image. PA1 a wavelength of light used for the photodetecting means is determined to be 750 nm or more, PA1 black toner forming the toner images mentioned above is one having a high transmission factor for the wavelength of 750 nm or more, and PA1 light-absorbing member absorbing exposure light mentioned above and an electrode member are provided on the outer circumferential surface of an photoreceptor facing the photodetecting means, PA1 which is a preferred embodiment of the invention. PA1 a process cartridge including the photoreceptor is detachable, and PA1 the photodetecting means mentioned above has functions as a toner detection means on the photoreceptor, a detection means for a transfer material, a reference position detection means of the photoreceptor and a light amount detection means of the imagewise exposure or an exposure means for neutralizing, PA1 which is a preferred embodiment of the invention.
However, the apparatus (A) has a drawback that a size of the apparatus become too large because the plural photoreceptor drums and the intermediate transfer members are required. The dimensions of the apparatus (B) can be made relatively smaller because the required number of each of the charging means, imagewise exposure means and photoreceptor is only a single, however, the apparatus (B) has a limitation that the size of a formed image is limited not to be larger than the area of the circumferential surface of the photoreceptor drum.
Apparatus (C) makes it possible to form images at high speed. In the apparatus (C), however, it is necessary to arrange a plurality of charging units, imagewise exposure means and developing units within the circumference of a photoreceptor, and it is also necessary to make a space between the imagewise exposure means and the developing unit large to avoid a fear that an optics system is contaminated by toner leaking from the developing unit that is close to the optics system and thereby image quality is deteriorated. Therefore, the photoreceptor diameter is inevitably large, resulting in a large apparatus, which is a contradiction.
For the purpose of preventing the disadvantages of the apparatus (C) mentioned above, there has been proposed an apparatus wherein a base substance of an photoreceptor is a transparent material which houses a plurality of imagewise exposure means and imagewise exposure is made on a photoreceptor layer on which images are formed through the aforesaid base substance (for example, Japanese Patent Publication Open to Public Inspection No. 307307/1993 (hereinafter referred to as Japanese Patent O.P.I Publication).
An apparatus based on the aforementioned proposal is one wherein there is provided a supporting member in which imagewise exposure means are arranged inside an photoreceptor, and exposure is conducted from the inside of the photoreceptor. In the conventional methods, there is one for obtaining control information of the image forming apparatus, such as toner density, gradation attribute, exposure amount compensation, jam detection of a transfer material, and reference position detection, by detecting a density of toner image formed on the photoreceptor. However, in the conventional methods, irradiation of light and detection of its reflection are conducted by a light detection means, which is provided outside the photoreceptor. In these methods, the light detection means is provided on the same side of a developer in relation to the photoreceptor; therefore, detection precision of the light detection means is deteriorated by the toner splash so that precise control of image forming apparatus can not be performed.